Conventionally known as a technique of an image forming apparatus capable of full-color image formation is a technique of converting image data for full-color printing into image data of a double color image composed of an achromatic color and a single chromatic color in order to print the double color image.
For example, Patent Literature 1 discloses the following technique. That is, color image data for a pixel whose saturation is lower than a predetermined value is corrected in such a manner that values of R, G, and B for the pixel are converted to be equal to each other. At the image formation based on the color image data having been subjected to the correction, an image composed of only K is formed for the pixel having the equal values of R, G, and B, while an image composed of any one of Y, M, and C is generated for the other pixel.
Further, Patent Literature 2 discloses a color image processing device including: achromatic color determining means that determines if a color to which a pixel is dedicated is an achromatic color or a chromatic color on the basis of multi-value data of each of the colors, R, G, and B; binarization process means that binarizes multi-value data of each of the colors R, G, and B and then outputs the binarized data; and color converting means that generates achromatic color data on the basis of the binarized data, for a pixel which has been determined to dedicate to an achromatic color, and then outputs the achromatic color data, while outputting the binarized data without modifying, for the pixel which has been determined to dedicate to a chromatic color.